The invention relates to an oscillation damping system for affecting an optical element in an imaging device, in particular deformation-decoupled mounts and manipulators in a projection illumination arrangement, in particular in a microlithographic projection exposure objective for the manufacture of semiconductors.
Especially in microlithographic projection exposure objective, very high imaging accuracies are required, oscillations introduced into the projection illumination arrangement, or its structures, having a very detrimental effect on the overall optics and therefore on the imaging accuracy.
In order to isolate the optical elements, e.g. lenses, from externally acting forces and deformations, it is known to connect the optical elements in deformation-decoupled fashion to outer structures, e.g. an outer mount. Such deformation-decoupled optical elements are, of necessity, relatively soft at the points of connection to the outer structure. This means that the natural frequencies become lower, which entails the risk that the optical elements can start to vibrate owing to externally excited oscillations, e.g. at machine frequencies, which in turn has a detrimental effect on the image quality of the illumination. For this reason, attempts have hitherto always been made to find a compromise between good deformation decoupling and a necessarily required minimum stiffness, so that the optical element does not experience oscillations.
It is an object of the present invention to provide a system with which the greatest possible deformation decoupling is achieved with correspondingly soft supporting or connecting points, but without externally acting oscillations having a detrimental effect on the optical element.
This object is achieved according to the invention with an optical mount unit comprising an optical element a mount holding said element, said mount comprising two units, which units are elastically coupled, and at least one active oscillation attenuator attached to said mount to affect oscillations of said elastically coupled units.
According to a further mode the present invention refers to an oscillation damping system for affecting an optical element in an objective, wherein the optical element is hold in a mount comprising two units, which units are elastically coupled, and where disturbing oscillations that occur being detected by sensors and then by actuators waves with same or at least similar frequencies and amplitudes of anti-phases to the disturbing oscillations generated and introduced in said mount.
According to a preferred mode the present invention refers to an oscillation damping system for affecting a lens in a microlithographic projection exposure objective.
According to the invention, oscillations are now detected by attenuators including sensors, whereupon actuators, especially piezoelectric elements, are activated in connection with a control system, especially an adaptronic control loop in such a way that waves directed in opposition, with respect to both amplitude and frequency, are activated by the actuators in such a way that the oscillations introduced from outside, in particular primary oscillations, are virtually completely or at least substantially eliminated.
In other words: the actuators generate waves with the same frequency and same amplitude, however of anti-phase, which results in an extinction of the disturbing oscillations. Of course, because of the many possibilities it is not generally possible to achieve complete extinction of all oscillations, although very strong damping can at least be ensured. Also oscillations or vibrations resulting from deformations can be damped with the attenuators.
In an advantageous configuration of the invention, the piezoelectric elements are provided as actuators in the form of thin plates, sheets or films, because in this way they can be integrated optimally into the structure.
A very advantageous configuration of the invention may consist in piezoelectric elements being provided as sensors, preferably in the form of thin plates, sheets or films.
The invention can be used particularly advantageously for deformation-decoupled mounts and manipulators. In this case, sensors are correspondingly distributed over the circumference, on a fixed or stationary outer ring and piezoelectric elements are provided as actuators, likewise distributed over the circumference, in or on an inner ring, which is elastically connected to the outer ring, or the optical element.
If the sensors and actuators are in this case arranged in the vicinity of joints, then oscillations or deformations induced externally via the outer mount can be deliberately counteracted by correspondingly activated actuators with anti-phase waves at the junction with the inner ring.
If, in a configuration according to the invention, the sensors are also designed as piezoelectric elements, then because of the deformations that occur at the joints and therefore length changes of the sensors that occur, the induced oscillations can be detected very exactly and oscillations that act in opposition, i.e. in anti-phase with same frequency and amplitude can be correspondingly generated.